1. Field of the Invention
This invention relates generally to electronic component assemblies wherein the electronic components are joined together by solder interconnections and, more particularly, to the rework of such assemblies by removing one component from another joined component.
2. Description of Related Art
Forming an electronic package assembly whereby an electrical component such as an integrated circuit chip is electrically and mechanically connected to a substrate such as a card, or board, another chip or another electronic part is well-known in the art. This technology is generally termed surface mount technology (SMT) and has gained acceptance as the preferred means of making electronic package assemblies. In one particular application, to which this application is directed to for convenience, multilayer ceramic components, as exemplified by integrated circuit chips, are joined to printed circuit cards or boards which boards are to be reworked by removing defective or unwanted chips from the board.
Multilayer ceramic electronic components are typically joined to printed circuit boards by soldering pads on a surface of one of the electronic components to corresponding soldering pads on the surface of the other component. Control Collapse Chip Connection is an interconnect technology developed by IBM as an alternative to wire bonding. This technology is generally known as C4 technology or flip chip packaging. Broadly stated, an integrated circuit chip is mounted above a board and pads on the chip are electrically and mechanically connected to corresponding pads on the board by a plurality of electrical connections such as solder bumps. The integrated circuit chips may be assembled in a solder array such as 10xc3x9710 array.
In the C4 interconnect technology, a relatively small solder bump is attached to the pads on one of the components being joined, typically to the chip. The electrical and mechanical interconnects are then formed by positioning the corresponding pads on the board to be joined adjacent the solder bumps on the chip and reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip bump pattern to the corresponding pads on the board.
A myriad of solder structures have been proposed for the surface mounting of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic pads exposed on the surface of the first electronic structure or substrate. A stencil printing operation is used to align the contact mask to the pads. The solder paste is reflowed in an inert atmosphere and homogenizes the pad and brings the solder into a spherical shape. The solder spheres on the substrate are then aligned to corresponding pads on the electronic structure or board to be connected thereto. After alignment, the substrate and board go through a reflow operation to melt the solder and create a solder bond between the corresponding pads on the substrate and other electronic component. The interconnection is typically in a form of a double truncated sphere and is termed a ball grid array (BGA). When the solder structure is in the form of a column, it is termed a column grid array (CGA).
It has now been proposed to use lead free solders in BGA/CGA interconnect schemes and these solders generally necessitate the use of higher temperatures during reflow attachment. Even higher than the attach temperatures are the temperatures required to do rework for removing defective modules and components, typically if the existing rework methods and tools are to be used. These excessive temperatures (typically above xcx9c250xc2x0 C.), can cause irreparable damage to the organic board substrate. Current methods require the use of such high temperatures to ensure that the solder joints reach the liquidus or eutectic temperature of the alloy (xcx9c221xc2x0 C. for eutectic Sn/Ag and xcx9c217xc2x0 C. for eutectic Sn/Ag/Cu). Rework of lead free alloys containing assemblies has become a critical issue in the qualification of this technology because of the high rework temperatures required.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for separating electronic components joined by solder connections and, in particular, lead-free solder connections in a BGA or CGA interconnect assembly such as a chip joined to a printed circuit board.
It is another object of the present invention to provide an apparatus for separating electronic components joined by solder connections and, in particular, joined together by lead-free solders in a BGA or CGA interconnect assemblies such as a chip joined to a printed circuit board.
It is yet another object of the present invention to provide an electronic component assembly reworked using the method and apparatus of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method for separating electronic components joined by solder connections comprising the steps of:
supplying an electronic component assembly having at least two components joined by a plurality of solder interconnections typically arranged in a row-by-row configuration and having a first thickness;
providing a cutting element having a thickness less than the first thickness of the solder interconnections;
heating the cutting element to a temperature sufficient to melt the solder at the point of contact when the cutting element is in contact with and forced against the solder interconnections;
positioning the heated cutting element adjacent a row of the solder interconnections;
applying a force to advance the heated cutting element through the row of solder interconnections whereby the heated cutting element engages and cuts through the solder interconnections and severs the row of solder interconnections;
continuing the above steps to cut and sever all the solder interconnections; and
separating the two components.
In a further aspect of the invention, the cutting element is a wire or a blade and is preferably moved sideways to the direction of travel of the cutting device either in one direction or in a reciprocating motion to provide a sawing effect as the cutting element is moved forward through the rows of solder interconnections.
In another aspect of the invention, the cutting element in the cutting device is a water knife which forms a high pressure water stream directed as a jet at the rows of solder interconnections to provide a cutting action to cut and sever the rows of solder interconnections.
In a further aspect of the invention, an apparatus is provided for separating electronic components joined by solder interconnections comprising:
securing means to hold an electronic assembly having at least two components joined by a plurality of solder interconnections arranged typically in a row-by-row configuration and having a first thickness;
a cutting element having a thickness less than the thickness of the solder interconnections;
a heater to heat the cutting element to a temperature sufficient to melt the solder at the point of contact when the cutting element is contacted with and forced forward against the solder interconnections;
positioning means to position the heated cutting element adjacent one of the rows of solder interconnections;
advancing means to force the heated cutting element against the row of solder interconnections and through the row of solder interconnections whereby the heated cutting element cuts and severs the row of solder interconnections; and
separating means to separate the two components when all the solder interconnections have been cut and severed by the heated cutting element.
In an additional aspect of the invention chips and other electronic components secured to a printed circuit board or other electronic component may be separated and/or reworked by removing one of the joined components e.g., a chip, and securing a new component, e.g., a chip to the other component, e.g., printed circuit board, using the method and apparatus of the invention.